1. Field of the Invention
This invention relates to the field of semiconductor wafer processing, and in particular to a method and apparatus for controlling the reaction chamber pressure in semiconductor wafer processing equipment.
2. Description of the Related Art
Fully automated wafer processing systems are currently in widespread commercial use. In such systems, wafers are robotically transported into and out of a reaction chamber or process chamber where, under a controlled evacuated environment, various processes are carried out. One example of such a wafer processing system is the TCP.TM. 9400 single-wafer plasma etching system manufactured by Lam Research Corporation.
In the case of plasma etching, semiconductor wafers are etched in the reaction chamber by exposing the wafer to ionized gas compounds (plasma) under very low pressures. Typically, during processing, pressures below one Torr need to be maintained in the reaction chamber. Process recipes consist of a series of steps controlling gas flow rates, chamber pressure, RF power, gap spacing, chamber temperature, and wafer temperature. Preprogrammed sets of process recipes are typically provided from the manufacturer. The operator may either select a programmed recipe or use an altered or customized recipe.
When the etching process starts, selected gases used for processing are mixed and introduced into the reaction chamber at rates according to the process recipe. RF power is delivered by a coil in the upper part of the reaction chamber and is tuned so as to ionize the process gases. RF power is also delivered to the wafer and is tuned so as to induce a DC bias on the wafer thereby controlling the direction and energy of ion bombardment of the wafer. During the etching process, the plasma reacts chemically with the wafer surface to remove material not covered by a mask. The plasma and RF electrical field are completely contained within the reaction chamber. An evacuation system continuously removes gases from the reaction chamber, and thereby maintains the desired pressure. The evacuation system typically comprises a turbo pump separated from the reaction chamber by a control gate valve. A pressure controller uses pressure data from a manometer in the reaction chamber to adjust the degree of closure position of the control gate valve. The pressure controller opens and closes the gate valve to increase and decrease the vacuum supplied from the turbo pump to the reaction chamber. In this way, the pressure controller attempts to maintain the desired pressure in the reaction chamber as the gas flow rates into the reaction chamber vary from one process step to the next.
The pressure controller compares the data from the reaction chamber manometer with the set point values programmed in the recipe. If either the process gas flow rate, or the desired pressure in the reaction chamber changes greatly from one process step to the next, the pressure controller may not be able to adjust the gate valve position appropriately. For example, if the next process step requires the gas to be introduced at much higher flow rates, the gate valve may initially over or under compensate, resulting in the wrong pressure in the reaction chamber at the beginning of the step. To alleviate this problem, currently available systems perform a learn procedure each time a new recipe is used. The learn procedure creates a table of the approximate gate valve positions required for each process step in the new recipe. During the learn procedure, dummy wafers are processed according to the process steps of the recipe, and the controller notes the gate valve positions required for each step.
One problem with the current systems is that a new learn procedure is required every time a new process recipe is used on a machine. Thus, when the operator alters the recipe by changing the set point pressure or gas flow of one of the process steps, he necessarily incurs the time and expense of deriving a new table. Running new learn procedures can be particularly time consuming when experimenting with new recipes.
Another problem with the current systems is that the results from the learn procedure will be invalid if any of the gas supply valves or instruments are incorrectly tuned or calibrated. For example, if one of the gas inflow valves is incorrectly calibrated during the learn procedure, the resulting table of gate positions will also be incorrect. Furthermore, the error in the table may not be detected until several wafers are incorrectly processed.
Another problem with the currently available systems is related to the movement of the gate valve itself. During processing, a certain amount of waste material generated from the etching process collects on the surface of the gate valve. Whenever the gate valve position is changed, particulate matter is released in the surrounding environment. Due to the extremely low operating pressures, back diffusion may carry the particles back upstream to the reaction chamber, where undesirable contamination of the wafer may occur.
A further limitation with the current systems is that the transition time or stabilization time may sometimes be undesirably long. The transition or stabilization time is the time required to stabilize the pressure in the reaction chamber to the pressure set point for a processing step. In particular, when the chamber starts out at relatively low pressure, such as at the beginning of processing, and the next step requires a relatively high pressure, such as 80 mTorr, the stabilization time typically takes around 20 seconds depending on the set point gas flow. According to the current practice, chamber stabilization is achieved by introducing the process gases into the reaction chamber at the set point flow rates required by the next processing step. Thus, in cases where the next processing step requires a relatively low flow, and the chamber pressure must be increased substantially, the stabilization time is often undesirably long.